Folding door systems can be either top hung or bottom rolling.
An advantage of a top hung system over a bottom rolling system is hardware used to mount a folding door in an overhead track, at a head of the system, is generally less likely to be subject to dirt and debris contamination. The hardware can accommodate larger rollers so the rolling performance is generally superior. As a result of accommodation of larger rollers, the top hung system can generally hold heavier panels.
A disadvantage of the top hung system is the head requires additional structural support to prevent the system from sagging under the weight. Also, panels of the folding door are more likely to require remedial re-adjustment should the head sag.
With the bottom rolling system, there is no need to provide additional support for the head as the weight load of the door is transferred directly into the floor. There is also no need to provide adjustment as the doors follow the contour of the floor.
However, rollers used in the bottom rolling system are more susceptible to contamination. Generally, the rollers have to be kept relatively small so as to keep the height of the threshold/sill to a minimum. However, smaller rollers tend to have a reduced load bearing capacity, generate more noise and require application of higher opening and closing forces to operate properly.